fuzzer: modernize FuzzedDataProvider conversions (#177794)

This change modernizes FuzzedDataProvider.h now that C++17+ is standard
in LLVM.
Replace the runtime if with if constexpr in ConvertUnsignedToSigned
Make the unsigned/signed comparison explicit by casting TS::max() to TU

GitOrigin-RevId: 1ce7a8159352e903f1dab50ff340fcd32a84cf63

FuzzedDataProvider.h

diff --git a/FuzzedDataProvider.h b/FuzzedDataProvider.h
index 33ffa8b..5fab0c4 100644
--- a/FuzzedDataProvider.h
+++ b/FuzzedDataProvider.h
@@ -380,13 +380,13 @@
   static_assert(!std::numeric_limits<TU>::is_signed,
                 "Source type must be unsigned.");
 
-  // TODO(Dor1s): change to `if constexpr` once C++17 becomes mainstream.
-  if (std::numeric_limits<TS>::is_modulo)
+  if constexpr (std::numeric_limits<TS>::is_modulo)
     return static_cast<TS>(value);
 
   // Avoid using implementation-defined unsigned to signed conversions.
   // To learn more, see https://stackoverflow.com/questions/13150449.
-  if (value <= std::numeric_limits<TS>::max()) {
+  constexpr auto TS_max = static_cast<TU>(std::numeric_limits<TS>::max());
+  if (value <= TS_max) {
     return static_cast<TS>(value);
   } else {
     constexpr auto TS_min = std::numeric_limits<TS>::min();